Problem: Which of these numbers is prime? ${4,\ 33,\ 51,\ 61,\ 75}$
Answer: A prime number has exactly two factors: 1 and itself. The factors of 4 are 1, 2, and 4. The factors of 33 are 1, 3, 11, and 33. The factors of 51 are 1, 3, 17, and 51. The factors of 61 are 1 and 61. The factors of 75 are 1, 3, 5, 15, 25, and 75. Thus, 61 is a prime number.